Replace it with a 4.5 power valve with the camshaft you're running; this will delay the opening slightly and give you a cleaner idle. Then turn in the idle-mixture screws until they bottom out lightly and back out all the screws one turn for a starting point. It's best to set the idle fuel-mixture screws with a vacuum gauge. Slowly turn in one of the idle screws and see if the idle vacuum goes up. This will indicate that the mixture is too rich. If the vacuum goes down when you turn the screw in, it's too lean. Find which direction you need to go and adjust all four screws equally until you have achieved the highest idle vacuum possible. The engine must be at operating temperature and the idle speed set before you adjust the mixture, and as you go, the idle speed may change and you must bring it back in line and go back to the mixture screws for the highest vacuum again. This should give you a good baseline to tune from. It should keep the plugs clean and give you a good test drive to figure out the jetting on the carb. Good luck and welcome back.
If you have a technical question for Kevin McClelland, send him an e-mail at: chevyhi@primedia.com.